Methodology for preparing a geminal-bis(difluoramino)-substituted nitrogenous heterocycle has been reported by Chapman et al. [Journal of Organic Chemistry 1998, 63, 1566-1570], who described the preparation of 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-bis(4-nitrobenzenesulfonyl)-1,5-diazocine; this intermediate has been converted to the corresponding nitramine, 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-dinitro-1,5-diazocine, given the acronym HNFX [Chapman et al., Journal of Organic Chemistry 1999, 64, 960-965]. Attempts have been made to convert 1,5-diacetyltetrahydro-1,5-diazocine-3,7(2H,6H)-dione to the corresponding 3,3,7,7-tetrakis(difluoramino)diazocine that led to facile addition of one difluoramine to one carbonyl site, but prolonged reaction to disrupt the stable transannular bridge which resulted only in degradation of the organic substrate, as reported by Chapman et al. [“Research in Energetic Compounds”, September 1991, Fluorochem Inc., Azusa, Calif., Report ONR-7-1; final report to the Office of Naval Research (Arlington, Va.) on Contract N00014-88-C-0536; described by Baum, K. et al. “Novel Approaches to the Synthesis of Fluorodinitromethane and Fluorodinitroethanol”, August 1993, Fluorochem Inc., Azusa, Calif., Report NRO-1-1; Chemical Abstracts 1995, 123, 339081r; NTIS Accession Number AD-A269158]. Using nitro as an alternative N-protecting group allowed further progress of the sequence to HNFX. Thus, nitrolysis of 3,7-diacetyl-5-(difluoramino)-9-oxa-3,7-diazabicyclo[3.3.1]nonan-1-ol produced an unprecedented nitrate ester of a hemiacetal, 5-(difluoramino)-3,7-dinitro-9-oxa-3,7-diazabicyclo[3.3.1]non-1-yl nitrate, which underwent slow spontaneous elimination of NO2 after workup. Difluoramination of the nitro-protected diazocine under conventional conditions allowed the formation of desired nitramine, but the yield was quite poor (˜1% in the last step) due to the known instability of nitramines in strong non-nitrating acids.
A 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-diazocine was desired as a potential precursor to 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-dinitro-1,5-diazocine (HNFX), the first example of a new class of compounds predicted to be potentially superior explosives or solid propellant oxidizers: gem-bis(difluoramino)-substituted heterocyclic nitramines. vic-Bis(difluoramino)-substituted primary nitramines and N-alkyl-N-(difluoraminomethyl)nitramines have been reported in prior literature. However, the synthesis of gem-bis(difluoramino)alkanes has required strongly acidic conditions, such as anhydrous sulfuric acid, difluorosulfamic acid, or fluorosulfonic acid, with which most nitramines are incompatible. In the first synthesis of HNFX, for example, the use of 5-(difluoramino)-3,7-dinitro-9-oxa-3,7-diazabicyclo[3.3.1]nonan-1-ol in a typical difluoramination reaction (difluoramine-difluorosulfamic acid-sulfuric acid) produced HNFX in only ˜1% yield. Therefore, the N-nitro component is preferably incorporated after difluoramination to produce gem-bis(difluoramino)alkyl components. A dilemma encountered in the preparation of a β,β-bis(difluoramino)-substituted heterocycle, such as a 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-diazocine, is that the nitrogen in most 1,3-diaminoacetone derivatives would be more basic than the ketone carbonyl (and oxygen in hemiaminal intermediates), thereby deactivating difluoramination via difluoramino-carbocations, unless the nitrogen is protected with a sufficiently electronegative protecting group to favorably affect this basicity [Chapman et al., Journal of Organic Chemistry 1998, 63, 1566-1570].
There exists a need in the art for 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-diazocinium salts for use as precursors to HNFX as well as to other members of the rare class of 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-diazocines.